1. Field of the Invention
The present invention relates to communications systems, and more particularly to a system and method for preventing signal quality degradation in a CDMA system.
2. Background of the Related Art
In a communications system using a CDMA modem system, interference signals or spurs occur during a call which tends to degrade calling quality. The degradation of calling quality in a related art system is explained as follows.
FIG. 1 illustrates a block diagram of a receiving circuit using a general CDM modem system. This circuit includes a low-noise amplifier (LNA) 111, a first band-pass filter (BPF) 112, a phase locked loop (PLL) 113, a voltage control oscillator (VCO) 114, a frequency mixer 115, a second band-pass filter 116, a demodulator 117, a low-pass filter(LPF) 118, an analog-digital converter 119, and a baseband modem 120. In the drawing, ‘I’, ‘Q’, ‘RF’, and ‘IF’ indicate coherent phase data, quadrature phase data, radio frequency signal, and intermediate signal respectively.
In the above-constituted CDMA receiving circuit, an RF signal input in to the antenna is amplified by low-noise amplifier 111, passed through first band-pass filter 112 to remove unnecessary wave components of the RF signal, and then input into frequency mixer 115. The RF signal is mixed with an output signal of the voltage control oscillator 114, which is controlled by phase locked loop 114. The frequency mixing converts the RF signal into an IF signal, which is then passed through second band-pass filter 116 and then input into demodulator 117. The demodulator transforms the IF signal into baseband I and Q signals and the demodulated signals pass through low-pass filter 118. Analog-digital converter 119 converts the signals into digital data which is then passed to baseband modem 120 so that an original signal may be regenerated
In the receiving circuit of the related art, an interference signal or spur may occur or exist on carrier feedthrough or calling. This signal or spur is undesirable because it has a severe influence on calling quality. Such an interference signal can be generated from various factors such as test radio waves, interference between the same or adjacent cells, frequency reuse, and the like. If the power of the interference signal source is greater than a level of the current call power, call disconnection and/or degradation of call quality may occur. Furthermore even if the power of the interference signal is smaller than the level of the current call power, the quality is often affected to some extent.
A need therefore exists for a system and method for improving call quality in a communications system, and more particularly ones which prevent signal degradation in a CDMA system as a result of spur or interference signals.